Hours before President Obama is set to deliver a major immigration speech, a key Republican senator blasted the president for reportedly opposing a requirement to shore up border security before legalizing up to 11 million illegal immigrants.

Sen. Marco Rubio, R-Fla., one of four Republican senators involved in a bipartisan effort to craft immigration reform legislation, warned the president Tuesday against taking such a position. It was the first sign since the senators unveiled their guidelines a day earlier of friction between the two efforts.

"I think that would be a terrible mistake," Rubio told Fox News. "We have a bipartisan group of senators that have agreed to that. For the president to try to move the goalposts on that specific requirement, as an example, does not bode well in terms of what his role's going to be in this or the outcome."

Rubio, a prominent conservative who is also Hispanic, is vital to the bipartisan effort on Capitol Hill. The senator, though, insisted that illegal immigrants not be allowed to obtain green cards -- let alone citizenship -- "until the enforcement stuff is in place."

"If that's not in the bill, I won't support it," he said.

Rubio was responding to reports that Obama, who is traveling to Las Vegas Tuesday to outline his immigration reform vision, does not want to make the legalization process contingent on increased border security.